The Red book Digital Audio logo is present in all discs and players ensuring compatibility

A CD player is an electronic device that plays audio compact discs. CD players are often a part of home stereo systems, car audio systems, and personal computers. They are also manufactured as portable devices. Modern units can play other formats in addition to PCM audio coding used in CDs, such as MP3, AAC and WMA. DJs often use players with an adjustable playback speed to alter the pitch and tempo of the music. CD playback functionality is also available on CD-ROM/DVD-ROM drive equipped computers as well as on DVD players and CD-ROM/DVD-ROM based game consoles.

The process of playing an Audio CD, touted as a digital audio storage medium, starts with the plastic polycarbonate disc, an analogue medium that contains the digitally encoded data.
The data is read out by loading the disc in the player's mechanism that scans the spiral data track using a laser beam. The tracking control is done by analogue servoamplifiers and then the high frequency analogue signal read from the disc is digitized, processed and decoded into analogue audio and digital control data which is used by the player to position the playback mechanism on the correct track, do the skip and seek functions and display track, time, index and, on newer players, title and artist information on a display placed in the front panel.[1]

Photodiode array on the Philips RAFOC single-beam tracking optical device used in many CDM optical assemblies

To read the data from the disc, a laser beam shines on the surface of the disc. Differences on the particular disc being played and in the loading mechanism makes the need of using a movable lens with a very close focal length to focus the light on the disc.

A low mass lens coupled to an electromagnetic coil is in charge of keeping focused the beam on the 600 nm wide data track.

When the player tries to read from a stop, it does a focus seek program that moves the lens up and down from the surface of the disc until a reflection is detected; when there is a reflection, the servo electronics lock in place keeping the lens in perfect focus while the disc rotates and changes its relative height from the optical block.
Different brands and models of optical assemblies use different methods of focus detection. On most players, the focus position detection is made using the difference in the current output of a block of four photodiodes. The photodiode block and the optics are arranged in such a way that a perfect focus projects a circular pattern on the block while a far or near focus projects an ellipse differing in the position of the long edge in north-south or west-southwest. That difference is the information that the servoamplifier uses to keep the lens at the proper reading distance during the playback operation, even if the disc is warped.[2]
Another servo mechanism in the player is in charge of keeping the focused beam centered on the data track.
Two optical pick-up designs exists, the original CDM series from Philips use a magnetic actuator mounted on a swing-arm to do coarse and fine tracking. Using only one laser beam and the 4 photodiode block, the servo knows if the track is centred by measuring side-by-side movement of the light of beam hitting on the block and corrects to keep the light on the centre.
The other design by Sony uses a diffraction grating to part the laser light into one main beam and two sub-beams. When focused, the two peripheral beams cover the border of the adjacent tracks a few micrometers apart from the main beam and reflect back on two photodiodes separated from the main block of four. The servo detects the RF signal being received on the peripheral receivers and the difference in output between these two diodes conform the tracking error signal that the system uses to keep the optics in the proper track. The tracking signal is fed to two systems, one integrated in the focus lens assembly can do fine tracking correction and the other system can move the entire optical assembly side by side to do coarse track jumps.

The sum of the output from the four photodiodes makes the RF or high frequency signal which is an electronic mirror of the pits and lands recorded on the disc.

The RF signal, when observed on an oscilloscope, has a characteristic "fish-eye" pattern and its usefulness in servicing the machine is paramount for detecting and diagnosing problems, and calibrating CD players for operation.

The first stage in the processing chain for the analog RF signal is digitizing it. Using various circuits like a simple comparator or a data slicer, the analog signal becomes a chain of two digital values, 1 and 0. This signal carries all the information in a CD and is modulated using a system called EFM (Eight-to-fourteen modulation).
The second stage is demodulating the EFM signal into a data frame that contains the audio samples, error correction parity bits, according with the CIRC error correction code, and control data for the player display and micro-computer. The EFM demodulator also decodes part of the CD signal and routes it to the proper circuits, separating audio, parity and control (subcode) data.
After demodulating, a CIRC error corrector takes each audio data frame, stores it in a SRAM memory and verifies that it has been read correctly, if it is not, it takes the parity and correction bits and fixes the data, then it moves it out to a DAC to be converted to an analog audio signal. If the data missing is enough to make recovery impossible, the correction is made by interpolating the data from subsequent frames so the missing part is not noticed. Each player has a different interpolation ability so if enough data frames are missing or unrecoverable, it may be impossible to fix by interpolation so an audio mute flag is raised to mute the DAC to avoid invalid data to be played back.
The Redbook standard dictates that, if there is invalid, erroneous or missing audio data, it cannot be output to the speakers as digital noise, it has to be muted.

The Audio CD format requires every player to have enough processing power to decode the CD data, this is made normally by application specific integrated circuits but they cannot work by themselves, they require a main microcomputer or microcontroller to orchestrate the entire machine.

Sony released its CD Player called the CDP-101[3] in 1982 with a slide-out tray design for the CD. As it was easy to manufacture and to use, most CD player manufacturers stayed with the tray style ever since. However there have been some notable exceptions.[4][5]

During the launch of the first prototype "Goronta" CD player[6] by Sony at the Japanese Audio Fair in 1982, Sony showcased the vertical loading design. Although the Sony prototype design was never put into volume production, the concept was for a time adopted for production by a number of early Japanese CD player manufacturers, including Alpine/Luxman, Matsushita under the Technics brand, Kenwood and Toshiba/Aurex. For the early vertical loading players, Alpine sourced their AD-7100 player designs for Luxman,[7] Kenwood and Toshiba (using their Aurex brand). Kenwood added their "Sigma Drive" outputs to this design as a modification. A picture of this early design can be seen on the Panasonic Web site.[8] The vertical loading is similar the one common in cassette decks, where the holder opens, and disc is dropped to it. The holder is closed manually by hand, by motor after pressing a button, or completely automatically. Some CD players combine vertical loading with slot loading due the disc being drawn further into the disc holder as it closes.

In 1983 Philips, at the US and European launch of the CD format, showcased the first top loading CD tray designs with their CD100 CD player.[9][10] (Philips audio products were sold as Magnavox in the US at the time.) The design had a clamp on the lid which meant the user had to close this over the CD when it was placed inside the machine. Later, Meridian introduced their MCD "high end" CD player,[11] with Meridian electronics in the Philips CD100 chassis.

Top-loading was adopted on various equipment designs such as mini systems and portable CD players, but among stereo component CD players, only a handful of top-loading models have been made. Examples include Luxman's D-500 and D-500X series[12] players and Denon's DP-S1,[13] both launched in 1993. Top-loading is also common in players intended for broadcast and live sound "DJ" use, such as Technics' SL-P50 (1984-1985) and Technics SL-P1200 (1986-1992). They more closely mimic the physical arrangement and ergonomics of record turntables used in those applications.

The Philips CD303 of 1983-1984 was the first player to adopt tray loading with sliding play mechanism. Basically as the tray came out to collect the CD, the entire player's transport system also came out as one unit. The Meridians 200 and 203 players were of this type and also the first to use a design in which the audio electronics were in a separate enclosure from the CD drive and pickup mechanism.

Slot loading is the preferred loading mechanism for car audio players. There is no tray that pops out, and a motor is used to assist disc insertion and removal. Some slot-loading mechanisms and changers can load and play back Mini-CDs without the need of an adapter but they may work with limited functionality (A disc changer will refuse to operate the changer until the Mini CD is removed for example). non-circular CDs cannot be used on such loaders because they cannot handle non-circular discs. Insertion of such media may become stuck and damage the mechanism.

The optical chip extracted from a CD player. The three dark rectangles are photosensitive, read the data from the disk and keep the beam focused. Electronic tracking, aided with the two photodiodes at the sides, keeps the laser beam centered on the middle of the data track.

Two types of optical tracking mechanisms exist:

The swing-arm mechanism, originally designed by Philips[14] – the lens moves at the end of an arm, in a manner similar to the tone arm assembly of a record player. Used in earliest Philips CD players and later replaced with cheaper radial mechanisms.

The radial mechanism, designed by Sony, which is the one used in most CD players nowadays – the lens moves on a radial rail being driven by a rotating gear from a motor or a linear magnetic assembly consisting of a solenoid mounted to the moving laser assembly, wound over a permanent magnetic field attached to the base of the mechanism. It is also known as three-beam linear tracking.

Philips one-beam laser assembly

The swing-arm mechanism has a distinctive advantage over the other in that it doesn't "skip" when the rail becomes dirty. The swing arm mechanisms tend to have a much longer life than their radial counterparts.[citation needed]

The main difference between the two mechanisms is the way they read the data from the disc.

The swing-arm mechanism uses a magnetic coil wound over a permanent magnet to provide the tracking movement to the laser assembly in a similar way a hard drive moves its head across the data tracks. It also uses another magnetic movement mechanism attached to the focusing lens to focus the laser beam on the disc surface. By operating the tracking or the focus actuators, the laser beam can be positioned on any part of the disc. This mechanism employs a single laser beam and a set of four photodiodes to read, focus and keep track of the data coming from the disc.[15]

Sharp laser optical assembly. All six focusing and tracking coils can be seen.

The linear tracking mechanism uses a motor and reduction gears to move the laser assembly radially across the tracks of the disc and it also has a set of six coils mounted in the focusing lens over a permanent magnetic field. One set of two coils moves the lens closer to the disc surface, providing the focusing motion, and the other set of coils moves the lens radially, providing a finer tracking motion. This mechanism uses the three-beam tracking method in which a main laser beam is used to read and focus the data track of the disc using three or four photodiodes, depending on the focus method, and two smaller beams read the adjacent tracks at each side to help the servo keep the tracking using two more "helper" photodiodes.[16]

A CD player has three major mechanical components : a drive motor, a lens system, and a tracking mechanism. The drive motor (also called spindle) rotates the disc between 200 and 500 revolutions per minute. The tracking mechanism moves the lens system along the spiral tracks in which information is encoded, and the lens assembly reads the information using a laser beam, typically produced by a laser diode. The laser reads information by focusing a beam on the CD, which is reflected off the disc's mirrored surface back to a photodiode array sensor. The sensor detects changes in the beam, and a digital processing chain interprets these changes as binary data. The data are processed, and eventually converted to sound using a digital-to-analog converter (DAC).

A TOC or Table of Contents is located after the "lead-in" area of the disc, which is located in an inner ring of the disc, and contains roughly five kilobytes of available space. It is the first information that the player reads when the disc is loaded in the player and contains information on the total number of audio tracks, the running time on the CD, the running time of each track, and other information such as ISRC and the format structure of the disc. The TOC is of such vital importance for the disc that if it is not read correctly by the player, the CD could not be played back. That's why it is repeated 3 times before the first music program starts.

The "lead out" area in the end (the outer peripheral) of the disc tells the player that disc has come to an end.

CD players can employ a number of ways to improve performance, or reduce component count or price. Features such as oversampling, one-bit DACs, dual DACs, interpolation (error correction), anti-skip buffering, digital and optical outputs are, or were, likely to be found. Other features improve functionality, such as track programming, random play and repeat, or direct track access. Yet others are related to the CD player's intended target, such as anti-skip for car and portable CD players, pitch control and queuing for a DJ's CD player, remote and system integration for household players. Description of some features follows:

Oversampling is a way to improve the performance of the low pass filter present at the output of most CD players. By using a higher sampling frequency, a multiple of the 44.1 kHz used by CD encoding, it can employ a filter with much lower requirements.

One-bit DACs were less expensive than other types of DACs, while providing similar performance.

Dual dacs were sometimes advertised as a feature because some of the early CD players used a single DAC, and switched it between channels. This required additional supporting circuits, possibly degrading sound quality.

Anti-skip or Antishock, is a way for the CD player to avoid interrupting the audio output when mechanical shock is experienced by the disc playback mechanism. It consists of an additional data processor and a RAM chip installed on the player that reads the disc at double speed and stores various frames of audio data in a RAM memory buffer for later decoding. Some players may compress the audio data prior to buffering to use less capacity (and less expensive) RAM chips. Most players can store about 44 seconds of audio data on a 16 mbit RAM chip.

A CD changer holds multiple Compact Discs, usually in a cartridge, revolving tray or carousel, and allows the user to access (or play) any of them, one at a time. CD changers are commonly found in home cinema systems, Home CD players, in cars and, less frequently, in computer equipment.

External cartridge CD changers have one or more cartridges that the user loads with up to twelve different CDs (depending on manufacturer) and then inserts into the CD changer. The CD changer can then remove one CD at a time for playing. This type of player is commonly found in vehicles because the user can switch easily between large amounts of media contained in different cartridges. However, more recent model years have phased out this feature in favor of flash media that can store more music on a lot less physical space, even CD players in general are being phased out from OEMs on newer model cars.

External, modular CD changer cartridges, over the years have been repurposed for some more passive uses, such as being a more rugged alternative to standard jewel cases for storing CD audio discs and DVD movies in.

Internal cartridge CD changers work on the same basic principle as external cartridge players, except the cartridge never leaves the CD player. This type of CD player accepts multiple CDs through a single slot and stores them internally.

A carousel-type CD changer consists of a circular platter that holds three or more CDs. Traditional carousel CD players hold three, five, or seven discs on a flat carousel tray. The carousel ejects to allow access to the CDs. Once back inside, the CD changer can rotate the carousel to access all of the CDs. Such carousel CD changers often allow the user to rotate the carousel while open, and change all CDs if no CDs are currently playing, or to eject the carousel while one CD is playing to change any CDs accessible from that position. Another type of carousel CD player is the mega-disc or "jukebox" CD players as they are also known. They usually hold anywhere from 50 to 300 discs. The mega-disc CD player holds its discs in a vertical position in slots that are located 360 degrees around the carousel. When a disc is selected to be played, the carousel rotates so that the disc can be picked up by a pickup mechanism and placed in CD playback unit. Mega-disc CD players generally have a means of entering in titles of the CDs stored inside them, such as telephone style letter input system found on the remote control, a full letter keypad on the unit, or by use of an external keyboard attached to the unit. Many units can also automatically obtain the title of a CDs if it contains CD-Text info stored on it.

CD-ROM changers existed in the early 1990s as expensive internal or external drives, often connecting through SCSI interfaces.
There were internal changers the size of a standard 5.25" drive that carried as few as 6 discs and external changers with capacity ranging from 50 discs to 200.
The high capacity changers used robotics to load and unload discs from big magazines into one of four built-in drives inside the machine. Two manufacturers of changers were NSM and Pioneer.

Changers were in a period when the mean hard drive capacity was measured in hundreds of megabytes. After exponential growth of hard drive storage density, a single inexpensive hard disk drive has many times the capacity of a CD, and changers have fallen out of use. Even flash memory can hold as many as more than 10 ISO images of CDs, and even as many as 5 ISO images of DVDs can be stored onto larger thumb drives.